A typical electric or hybrid-electric vehicle derives power for propulsion from a traction battery. During accidents and when power is not required from the traction battery, the terminals of the battery may be disconnected from other components. Isolating the traction battery from the high-voltage bus of the vehicle is typically achieved using a main contactor. The main contactor may be damaged when current flows during closure due to large inrush currents of devices attached to the high-voltage bus. To eliminate closing the main contactor while large inrush currents are present, most systems employ a pre-charge circuit to limit the current flowing to the high-voltage bus at power up. A typical pre-charge circuit may consist of a pre-charge contactor and a resistor.
The resistor of the pre-charge circuit limits the current flow to the high-voltage bus from the traction battery. Once the high-voltage bus reaches a certain voltage, the main contactor may be closed. The main contactor closes with a much smaller voltage across the contactor that reduces current flow which minimizes arcing and possible damage to the main contactor. The pre-charge components are only used during vehicle start up. However, to prevent damage due to rapid on-off cycling of the ignition, pre-charge components may be designed to handle a continuous usage profile.